Filename: 179-TLS-cert-and-parameter-normalization.txt
Title: TLS certificate and parameter normalization
Author: Jacob Appelbaum, Gladys Shufflebottom
Created: 16-Feb-2011
Status: Closed
Target: 0.2.3.x


        Draft spec for TLS certificate and handshake normalization


                                    Overview

     STATUS NOTE:

     This document is implemented in part in 0.2.3.x, deferred in part, and
     rejected in part.  See indented bracketed comments in individual
     sections below for more information. -NM

Scope

This is a document that proposes improvements to problems with Tor's
current TLS (Transport Layer Security) certificates and handshake that will
reduce the distinguishability of Tor traffic from other encrypted traffic that
uses TLS.  It also addresses some of the possible fingerprinting attacks
possible against the current Tor TLS protocol setup process.

Motivation and history

Censorship is an arms race and this is a step forward in the defense
of Tor.  This proposal outlines ideas to make it more difficult to
fingerprint and block Tor traffic.

Goals

This proposal intends to normalize or remove easy-to-predict or static
values in the Tor TLS certificates and with the Tor TLS setup process.
These values can be used as criteria for the automated classification of
encrypted traffic as Tor traffic. Network observers should not be able
to trivially detect Tor merely by receiving or observing the certificate
used or advertised by a Tor relay. I also propose the creation of
a hard-to-detect covert channel through which a server can signal that it
supports the third version ("V3") of the Tor handshake protocol.

Non-Goals

This document is not intended to solve all of the possible active or passive
Tor fingerprinting problems. This document focuses on removing distinctive
and predictable features of TLS protocol negotiation; we do not attempt to
make guarantees about resisting other kinds of fingerprinting of Tor
traffic, such as fingerprinting techniques related to timing or volume of
transmitted data.

                                Implementation details


Certificate Issues

The CN or commonName ASN1 field

Tor generates certificates with a predictable commonName field; the
field is within a given range of values that is specific to Tor.
Additionally, the generated host names have other undesirable properties.
The host names typically do not resolve in the DNS because the domain
names referred to are generated at random. Although they are syntatically
valid, they usually refer to domains that have never been registered by
any domain name registrar.

An example of the current commonName field: CN=www.s4ku5skci.net

An example of OpenSSL’s asn1parse over a typical Tor certificate:

   0:d=0  hl=4 l= 438 cons: SEQUENCE
    4:d=1  hl=4 l= 287 cons: SEQUENCE
    8:d=2  hl=2 l=   3 cons: cont [ 0 ]
   10:d=3  hl=2 l=   1 prim: INTEGER           :02
   13:d=2  hl=2 l=   4 prim: INTEGER           :4D3C763A
   19:d=2  hl=2 l=  13 cons: SEQUENCE
   21:d=3  hl=2 l=   9 prim: OBJECT            :sha1WithRSAEncryption
   32:d=3  hl=2 l=   0 prim: NULL
   34:d=2  hl=2 l=  35 cons: SEQUENCE
   36:d=3  hl=2 l=  33 cons: SET
   38:d=4  hl=2 l=  31 cons: SEQUENCE
   40:d=5  hl=2 l=   3 prim: OBJECT            :commonName
   45:d=5  hl=2 l=  24 prim: PRINTABLESTRING   :www.vsbsvwu5b4soh4wg.net
   71:d=2  hl=2 l=  30 cons: SEQUENCE
   73:d=3  hl=2 l=  13 prim: UTCTIME           :110123184058Z
   88:d=3  hl=2 l=  13 prim: UTCTIME           :110123204058Z
  103:d=2  hl=2 l=  28 cons: SEQUENCE
  105:d=3  hl=2 l=  26 cons: SET
  107:d=4  hl=2 l=  24 cons: SEQUENCE
  109:d=5  hl=2 l=   3 prim: OBJECT            :commonName
  114:d=5  hl=2 l=  17 prim: PRINTABLESTRING   :www.s4ku5skci.net
  133:d=2  hl=3 l= 159 cons: SEQUENCE
  136:d=3  hl=2 l=  13 cons: SEQUENCE
  138:d=4  hl=2 l=   9 prim: OBJECT            :rsaEncryption
  149:d=4  hl=2 l=   0 prim: NULL
  151:d=3  hl=3 l= 141 prim: BIT STRING
  295:d=1  hl=2 l=  13 cons: SEQUENCE
  297:d=2  hl=2 l=   9 prim: OBJECT            :sha1WithRSAEncryption
  308:d=2  hl=2 l=   0 prim: NULL
  310:d=1  hl=3 l= 129 prim: BIT STRING

I propose that we match OpenSSL's default self-signed certificates. I hypothesise
that they are the most common self-signed certificates. If this turns out not
to be the case, then we should use whatever the most common turns out to be.

Certificate serial numbers

Currently our generated certificate serial number is set to the number of
seconds since the epoch at the time of the certificate's creation. I propose
that we should ensure that our serial numbers are unrelated to the epoch,
since the generation methods are potentially recognizable as Tor-related.

Instead, I propose that we use a randomly generated number that is
subsequently hashed with SHA-512 and then truncate the data to eight bytes[1].

Random sixteen byte values appear to be the high bound for serial number as
issued by Verisign and DigiCert.  RapidSSL appears to be three bytes in length.
Others common byte lengths appear to be between one and four bytes. The default
OpenSSL certificates are eight bytes and we should use this length with our
self-signed certificates.

This randomly generated serial number field may now serve as a covert channel
that signals to the client that the OR will not support TLS renegotiation; this
means that the client can expect to perform a V3 TLS handshake setup.
Otherwise, if the serial number is a reasonable time since the epoch, we should
assume the OR is using an earlier protocol version and hence that it expects
renegotiation.

We also have a need to signal properties with our certificates for a possible
v3 handshake in the future. Therefore I propose that we match OpenSSL default
self-signed certificates (a 64-bit random number), but reserve the two least-
significant bits for signaling. For the moment, these two bits will be zero.

This means that an attacker may be able to identify Tor certificates from default
OpenSSL certificates with a 75% probability.

As a security note, care must be taken to ensure that supporting this
covert channel will not lead to an attacker having a method to downgrade client
behavior. This shouldn't be a risk because the TLS Finished message hashes over
all the bytes of the handshake, including the certificates.

     [Randomized serial numbers are implemented in 0.2.3.9-alpha. We probably
     shouldn't do certificate tagging by a covert channel in serial numbers,
     since doing so would mean we could never have an externally signed
     cert. -NM]

Certificate fingerprinting issues expressed as base64 encoding

It appears that all deployed Tor certificates have the following strings in
common:

MIIB
CCA
gAwIBAgIETU
ANBgkqhkiG9w0BAQUFADA
YDVQQDEx
3d3cu

As expected these values correspond to specific ASN.1 OBJECT IDENTIFIER (OID)
properties (sha1WithRSAEncryption, commonName, etc) of how we generate our
certificates.

As an illustrated example of the common bytes of all certificates used within
the Tor network within a single one hour window, I have replaced the actual
value with a wild card ('.') character here:

-----BEGIN CERTIFICATE-----
MIIB..CCA..gAwIBAgIETU....ANBgkqhkiG9w0BAQUFADA.M..w..YDVQQDEx.3
d3cu............................................................
................................................................
................................................................
................................................................
................................................................
................................................................
................................................................
................................................................
........................... <--- Variable length and padding
-----END CERTIFICATE-----

This fine ascii art only illustrates the bytes that absolutely match in all
cases.  In many cases, it's likely that there is a high probability for a given
byte to be only a small subset of choices.

Using the above strings, the EFF's certificate observatory may trivially
discover all known relays, known bridges and unknown bridges in a single SQL
query.  I propose that we ensure that we test our certificates to ensure that
they do not have these kinds of statistical similarities without ensuring
overlap with a very large cross section of the internet's certificates.

Certificate dating and validity issues

TLS certificates found in the wild are generally found to be long-lived;
they are frequently old and often even expired. The current Tor certificate
validity time is a very small time window starting at generation time and
ending shortly thereafter, as defined in or.h by MAX_SSL_KEY_LIFETIME
(2*60*60).

I propose that the certificate validity time length is extended to a period of
twelve Earth months, possibly with a small random skew to be determined by the
implementer. Tor should randomly set the start date in the past or some
currently unspecified window of time before the current date. This would
more closely track the typical distribution of non-Tor TLS certificate
expiration times.

The certificate values, such as expiration, should not be used for anything
relating to security; for example, if the OR presents an expired TLS
certificate, this does not imply that the client should terminate the
connection (as would be appropriate for an ordinary TLS implementation).
Rather, I propose we use a TOFU style expiration policy - the certificate
should never be trusted for more than a two hour window from first sighting.

This policy should have two major impacts. The first is that an adversary will
have to perform a differential analysis of all certificates for a given IP
address rather than a single check. The second is that the server expiration
time is enforced by the client and confirmed by keys rotating in the consensus.

The expiration time should not be a fixed time that is simple to calculate by
any Deep Packet Inspection device or it will become a new Tor TLS setup
fingerprint.

   [Deferred and needs revision; see proposal XXX. -NM]

Proposed certificate form

The following output from openssl asn1parse results from the proposed
certificate generation algorithm. It matches the results of generating a
default self-signed certificate:

    0:d=0  hl=4 l= 513 cons: SEQUENCE          
    4:d=1  hl=4 l= 362 cons: SEQUENCE          
    8:d=2  hl=2 l=   9 prim: INTEGER           :DBF6B3B864FF7478
   19:d=2  hl=2 l=  13 cons: SEQUENCE          
   21:d=3  hl=2 l=   9 prim: OBJECT            :sha1WithRSAEncryption
   32:d=3  hl=2 l=   0 prim: NULL              
   34:d=2  hl=2 l=  69 cons: SEQUENCE          
   36:d=3  hl=2 l=  11 cons: SET               
   38:d=4  hl=2 l=   9 cons: SEQUENCE          
   40:d=5  hl=2 l=   3 prim: OBJECT            :countryName
   45:d=5  hl=2 l=   2 prim: PRINTABLESTRING   :AU
   49:d=3  hl=2 l=  19 cons: SET               
   51:d=4  hl=2 l=  17 cons: SEQUENCE          
   53:d=5  hl=2 l=   3 prim: OBJECT            :stateOrProvinceName
   58:d=5  hl=2 l=  10 prim: PRINTABLESTRING   :Some-State
   70:d=3  hl=2 l=  33 cons: SET               
   72:d=4  hl=2 l=  31 cons: SEQUENCE          
   74:d=5  hl=2 l=   3 prim: OBJECT            :organizationName
   79:d=5  hl=2 l=  24 prim: PRINTABLESTRING   :Internet Widgits Pty Ltd
  105:d=2  hl=2 l=  30 cons: SEQUENCE          
  107:d=3  hl=2 l=  13 prim: UTCTIME           :110217011237Z
  122:d=3  hl=2 l=  13 prim: UTCTIME           :120217011237Z
  137:d=2  hl=2 l=  69 cons: SEQUENCE          
  139:d=3  hl=2 l=  11 cons: SET               
  141:d=4  hl=2 l=   9 cons: SEQUENCE          
  143:d=5  hl=2 l=   3 prim: OBJECT            :countryName
  148:d=5  hl=2 l=   2 prim: PRINTABLESTRING   :AU
  152:d=3  hl=2 l=  19 cons: SET               
  154:d=4  hl=2 l=  17 cons: SEQUENCE          
  156:d=5  hl=2 l=   3 prim: OBJECT            :stateOrProvinceName
  161:d=5  hl=2 l=  10 prim: PRINTABLESTRING   :Some-State
  173:d=3  hl=2 l=  33 cons: SET               
  175:d=4  hl=2 l=  31 cons: SEQUENCE          
  177:d=5  hl=2 l=   3 prim: OBJECT            :organizationName
  182:d=5  hl=2 l=  24 prim: PRINTABLESTRING   :Internet Widgits Pty Ltd
  208:d=2  hl=3 l= 159 cons: SEQUENCE          
  211:d=3  hl=2 l=  13 cons: SEQUENCE          
  213:d=4  hl=2 l=   9 prim: OBJECT            :rsaEncryption
  224:d=4  hl=2 l=   0 prim: NULL              
  226:d=3  hl=3 l= 141 prim: BIT STRING        
  370:d=1  hl=2 l=  13 cons: SEQUENCE          
  372:d=2  hl=2 l=   9 prim: OBJECT            :sha1WithRSAEncryption
  383:d=2  hl=2 l=   0 prim: NULL              
  385:d=1  hl=3 l= 129 prim: BIT STRING        

    [Rejected pending more evidence; this pattern is trivially detectable,
    and there is just not enough reason at the moment to think that this
    particular certificate pattern is common enough for sites that matter
    that the censors wouldn't be willing to block it. -NM]

Custom Certificates

It should be possible for a Tor relay operator to use a specifically supplied
certificate and secret key. This will allow a relay or bridge operator to use a
certificate signed by any member of any geographically relevant certificate
authority racket; it will also allow for any other user-supplied certificate.
This may be desirable in some kinds of filtered networks or when attempting to
avoid attracting suspicion by blending in with the TLS web server certificate
crowd.

    [Deferred; see proposal XXX]

Problematic Diffie–Hellman parameters

We currently send a static Diffie–Hellman parameter, prime p (or “prime p
outlaw”) as specified in RFC2409 as part of the TLS Server Hello response.

The use of this prime in TLS negotiations may, as a result, be filtered and
effectively banned by certain networks. We do not have to use this particular
prime in all cases.

While amusing to have the power to make specific prime numbers into a new class
of numbers (cf. imaginary, irrational, illegal [3]) - our new friend prime p
outlaw is not required.

The use of this prime in TLS negotiations may, as a result, be filtered and
effectively banned by certain networks. We do not have to use this particular
prime in all cases.

I propose that the function to initialize and generate DH parameters be
split into two functions.

First, init_dh_param() should be used only for OR-to-OR DH setup and
communication. Second, it is proposed that we create a new function
init_tls_dh_param() that will have a two-stage development process.

The first stage init_tls_dh_param() will use the same prime that
Apache2.x [4] sends (or “dh1024_apache_p”), and this change should be
made immediately. This is a known good and safe prime number (p-1 / 2
is also prime) that is currently not known to be blocked.

The second stage init_tls_dh_param() should randomly generate a new prime on a
regular basis; this is designed to make the prime difficult to outlaw or
filter.  Call this a shape-shifting or "Rakshasa" prime.  This should be added
to the 0.2.3.x branch of Tor. This prime can be generated at setup or execution
time and probably does not need to be stored on disk. Rakshasa primes only
need to be generated by Tor relays as Tor clients will never send them. Such
a prime should absolutely not be shared between different Tor relays nor
should it ever be static after the 0.2.3.x release.

As a security precaution, care must be taken to ensure that we do not generate
weak primes or known filtered primes. Both weak and filtered primes will
undermine the TLS connection security properties. OpenSSH solves this issue
dynamically in RFC 4419 [5] and may provide a solution that works reasonably
well for Tor. More research in this area including the applicability of
Miller-Rabin or AKS primality tests[6] will need to be analyzed and probably
added to Tor.

      [Randomized DH groups are implemented in 0.2.3.9-alpha. -NM]

Practical key size

Currently we use a 1024 bit long RSA modulus. I propose that we increase the
RSA key size to 2048 as an additional channel to signal support for the V3
handshake setup.  2048 appears to be the most common key size[0] above 1024.
Additionally, the increase in modulus size provides a reasonable security boost
with regard to key security properties.

The implementer should increase the 1024 bit RSA modulus to 2048 bits.

     [Deferred and needs performance analysis.  See proposal
     XXX. Additionally, DH group strength seems far more crucial. Still, this
     is out-of-scope for a "normalization" question. -NM]

Possible future filtering nightmares

At some point it may cost effective or politically feasible for a network
filter to simply block all signed or self-signed certificates without a known
valid CA trust chain. This will break many applications on the internet and
hopefully, our option for custom certificates will ensure that this step is
simply avoided by the censors.

The Rakshasa prime approach may cause censors to specifically allow only
certain known and accepted DH parameters.


Appendix: Other issues

What other obvious TLS certificate issues exist? What other static values are
present in the Tor TLS setup process?

[0] http://archives.seul.org/or/dev/Jan-2011/msg00051.html
[1] http://archives.seul.org/or/dev/Feb-2011/msg00016.html
[2] http://archives.seul.org/or/dev/Feb-2011/msg00039.html
[3] To be fair this is hardly a new class of numbers. History is rife with
    similar examples of inane authoritarian attempts at mathematical secrecy.
    Probably the most dramatic example is the story of the pupil Hipassus of
    Metapontum, pupil of the famous Pythagoras, who, legend goes, proved the
    fact that Root2 cannot be expressed as a fraction of whole numbers (now
    called an irrational number) and was assassinated for revealing this
    secret.  Further reading on the subject may be found on the Wikipedia:
    http://en.wikipedia.org/wiki/Hippasus

[4] httpd-2.2.17/modules/ss/ssl_engine_dh.c
[5] http://tools.ietf.org/html/rfc4419
[6] http://archives.seul.org/or/dev/Jan-2011/msg00037.html
